The present invention relates to a process for preparing aqueous polymer dispersions, where first of all compounds having at least one ethylenically unsaturated group (monomers) are polymerized by the method of free-radical aqueous emulsion polymerization to give a polymer 1, in disperse distribution in the aqueous medium, and then, in one or more successive polymerization stages i, further monomers are subjected to free-radical polymerization in the presence of the polymer 1 that is in disperse distribution in the aqueous medium, with the proviso that
a) the polymer 1 has a glass transition temperature Tg1;
b) the monomers polymerized in each of the polymerization stages i are such that isolated random copolymerization of these monomers would give a polymer i whose glass transition temperature tends with increasing molecular weight toward the limit value Tgi;
c) for each Tgi the difference Tgixe2x88x92Tg1 is xe2x89xa710xc2x0 C.;
d) the amount of the monomers polymerized to prepare the polymer 1 in disperse distribution, based on the amount of all of the monomers polymerized in the process, is  greater than 85 (preferably xe2x89xa787 and particularly preferably xe2x89xa790) and xe2x89xa699.9% by weight;
e) the total amount of the monomers polymerized in all polymerization stages i, based on the total amount of all of the monomers polymerized in the process, is xe2x89xa70.1% by weight and  less than 15 (preferably xe2x89xa613 and particularly preferably xe2x89xa610) % by weight;
f) in each of the polymerization stages i, the addition of the monomers to be polymerized in this polymerization stage i to the polymerization vessel is made such that at no time up to the end of the addition does the degree of polymerization Ui of monomers to be polymerized in stage i and monomers already added to the polymerization vessel exceed 50 mol-%;
g) the total amount of the monomers polymerized to prepare the polymer 1, apart from monomers having two conjugated ethylenically unsaturated groups, does not embrace more than 5% by weight of monomers having more than one ethylenically unsaturated group.
Ui here is defined as the percentage of those monomers that have already been added to the polymerization vessel for polymerization in the i-th polymerization stage whose polymerization has already taken place at the time of the addition of further monomers to be polymerized in the polymerization stage i.
A polymerization stage i is defined as the period of time within which the rate of polymerization (the molar quantity of monomers polymerized per unit time), starting from a very low level, first of all increases and then, after having passed through a maximum, decreases again either as a result of consumption of the monomers to be polymerized in this stage and/or of the polymerization initiator, and/or as a result of deliberate outside interference (e.g. reducing the polymerization temperature, adding polymerization inhibitors), before increasing again with the commencement of the next polymerization stage.
The present invention additionally relates to the aqueous polymer dispersions obtainable using the novel process and to their use for coating, bonding, sealing or impregnating, preferance being given to their use as binders for moldings or coating compositions comprising finely divided mineral and/or organic substances (fillers and/or pigments) and as contact adhesives with improved cohesion.
Aqueous polymer dispersions are fluid systems whose disperse phase, in the aqueous dispersion medium, comprises polymer coils (polymer particles) in substantially stable disperse distribution.
Like polymer solutions when the solvent is evaporated, aqueous polymer dispersions have the property, when the aqueous dispersion medium is evaporated, of forming polymer films, which is why aqueous polymer dispersions are much employed as binders, for example for paints or compositions for coating wood, roofs or leather, and as contact adhesives.
In numerous cases, both such polymer films, or the coating compositions comprising them, and the substrate which is coated with them, are subject to great fluctuations in humidity and/or temperature, resulting in extreme expansions and contractions of the films and/or of the substrate on which they adhere; in other words the films are often subject to high stresses which the films can only withstand undamaged (without fracturing or cracking) when a very large amount of energy is required to induce their fracture.
An appropriate measure of the fracture energy to be performed on a polymer film is, in accordance with EP-B 264 903, the product of elongation at break and tear strength of the polymer film. Accordingly, advantageous polymer films are those whose product of elongation at break and tear strength is high; among the polymer films having a high value of this product, preference is given to those which simultaneously feature both a high elongation at break and a high tear strength. The latter objective is particularly difficult to realize, since it is usually the case that measures to improve the tear strength impair the elongation at break, and vice versa (these mechanical properties of a polymer film are usually determined at a temperature above its Tg).
It is an object of the present invention, therefore, to provide a process whose application, on the one hand, permits an increase in the energy required to bring about the fracture of a film of an aqueous polymer dispersion (xe2x80x9cof a polymer 1xe2x80x9d) and, on the other hand, at the same time ensures both a very high elongation break and a very high tear strength of the polymer film.
We have found that this object is achieved by the process defined at the outset for preparing an aqueous polymer dispersion.
Pressure-sensitive adhesives (PSAs) form a permanent tacky film which at room temperature, even under slight pressure, sticks to a very wide variety of surfaces. Pressure-sensitive adhesives are used to produce self-adhesive products such as self-adhesive labels, tapes and films. Products of this kind are very simple to use and make it possible to work rapidly when bonding. In contrast to contact adhesive compositions, no ventilation times are necessary. Moreover, there is no xe2x80x9copen timexe2x80x9d within which the adhesive bond must be implemented. The quality of a self-adhesive article depends essentially on whether the internal strength (cohesion) and the sticking of the adhesive film on the surface that is to be bonded (adhesion) are set properly in relation to one another in accordance with the utility.
In the case of pressure-sensitive adhesive labels, for example, the level of cohesion must be sufficient for no stringing and no emergence of glue at the edges to occur in the course of stamping and cutting, since otherwise the cutting tools become soiled and the cut faces sticky. At the same time, the adhesion should be at a high level in order to obtain good sticking on the substrate that is to be bonded.
In general it is not possible to optimize adhesion and cohesion independently of one another. There is a desire for measures which either lift the level of both properties or, at least, maintain one property unchanged while improving the other.
U.S. Pat. No. 4,939,190 and EP 215 241 disclose pressure-sensitive adhesives comprising polymers of multistage composition. However, these polymers still do not show the desired level of adhesion or cohesion, or the desired adhesion/cohesion relationship.
Surprisingly, it has also been found that certain polymers, obtainable in accordance with the invention, are excellent pressure-sensitive adhesives having high cohesion and high adhesion. These polymers are defined in more detail below.
Polymerization processes similar to that of the invention are known from Polymer, 1985, Vol. 26, pp. 1359 to 1364, from J. of Appl. Polymer Science, Vol. 56, pp. 793 to 802 (1995), from EP-A 359 562, from EP-A 522 789 and from EP-A 187 505. Disadvantages of these processes, however, are that they are unable to establish satisfactorily the mechanical property profile of the resulting polymer films, as desired in accordance with the invention, and/or that in the subsequent polymerization stages they necessarily require the copolymerization of monomers containing polyethylenic (non-conjugated) unsaturation, which are comparatively expensive.
Moreover, the literature discloses numerous at least two-stage processes of free-radical aqueous emulsion polymerization (e.g. U.S. Pat. No. 3,562,235 and EP-A 600 478) in which the addition to the polymerization vessel of the monomers to be polymerized in the various polymerization stages is made at the rate at which they are consumed and thus in such a way that for the majority of the period up to the end of the addition the degree of polymerization of the monomers to be polymerized in one stage and of the monomers already added to the polymerization vessel is above 80 mol-%. Disadvantages of these processes, however, are that they are in general unable to establish satisfactorily the mechanical property profile of the resulting polymer films, as desired in accordance with the invention, and/or that they are comparatively complex to implement.
An essential feature of the novel process is that monomers which are polymerized in a stage i are added to the polymerization vessel such that a significant proportion thereof is able to dissolve, prior to its polymerization, in the polymer particles which are already present in the polymerization vessel, in disperse distribution in the aqueous medium, and is thus polymerized in the form in which it is dissolved in these dispersed polymer particles (ie. after swelling the dispersed polymer particles).
Preferably, therefore, the procedure adopted in accordance with the invention is such that in each polymerization stage i the addition of the monomers to be polymerized in this polymerization stage i to the polymerization vessel is made such that at no point up to the end of the addition does the degree of polymerization Ui of the monomers to be polymerized in the stage i and of the monomers already added to the polymerization vessel exceed 40 mol-%. Preferably, the abovementioned Ui up to the end of the addition is xe2x89xa630 mol-% and, very preferably, xe2x89xa620 or xe2x89xa610 mol-% or xe2x89xa65 mol-%. It is particularly advantageous, when adding the monomers to be polymerized in a stage i to the polymerization vessel, substantially to interrupt the polymerization, in other words to carry out the polymerization of the monomers to be polymerized in stage i only after having added the total amount of the monomers to be polymerized in stage i to the polymerization vessel. Here, the total amount of the monomers to be polymerized in stage i can, as always within the context of the novel process, be added to the polymerization vessel either all at once or in portions. As already mentioned, the polymerization can be interrupted by measures such as temperature reduction, addition of polymerization inhibitors (free-radical scavengers such as hydroquinone, for example), consumption of initiator, etc.
The swelling of the polymer particles already present in disperse distribution in the aqueous medium, in the polymerization vessel, by the monomers to be polymerized in a polymerization stage i is in general made easier in that these monomers are not pre-emulsified in the aqueous medium but are added in pure form. Furthermore, it is normally and in accordance with the invention of advantage if the addition, in the case of substantially interrupted polymerization, is made at elevated temperature, for example at from 50 to 95xc2x0 C. or from 60 to 80xc2x0 C. Elevated temperatures of this kind normally accelerate the desired dissolution and swelling process. The latter is also positively influenced by providing sufficient time. In other words, if necessary, the monomers to be polymerized in a polymerization stage i are added to the polymerization vessel and the contents of the polymerization vessel are left alone with polymerization interrupted, preferably with stirring and at elevated temperature, for a prolonged period (from several minutes through several hours up to several days). Such a measure is particularly beneficial in many cases when the polymer 1 includes, in copolymerized form, monomers having two nonconjugated ethylenically unsaturated groups. The latter crosslink the polymer chains present in a polymer particle, thereby making the desired swelling more difficult.
In accordance with the invention, therefore, it is generally advantageous if the total amount of the monomers polymerized to prepare the polymer 1, apart from monomers having two conjugated ethylenically unsaturated groups, does not embrace more than 2% by weight of monomers having more than one ethylenically unsaturated group. It is normally of advantage in accordance with the invention for the polymer 1 to contain in copolymerized form no such crosslinking monomers whatsoever.
Although monomers, such as butadiene, that have two conjugated ethylenically unsaturated groups normally induce almost no pronounced crosslinking within a free-radical aqueous emulsion polymerization (the reactivity of the second ethylenically unsaturated group is normally lowered), it is advantageous in accordance with the invention if the polymer 1 contains in copolymerized form no monomers having two conjugated ethylenically unsaturated groups. The same applies to the polymerization stages i.
It is also preferred if the monomers polymerized in the polymerization stages i comprise no monomers having more than one nonconjugated, ethylenically unsaturated group; in other words, these monomers preferably comprise no crosslinking agent.
In the novel process, the preparation of the polymer 1 is preferably followed by only one polymerization stage i. In that case, some of the monomers to be polymerized in the one polymerization stage i may already have been added to the polymerization vessel as part of the preparation of the polymer 1. This is the case, for example, when the free-radical aqueous emulsion polymerization to prepare the polymer 1 is deliberately broken off before the monomers added to the polymerization vessel have been incorporated completely by polymerization. The remaining monomers then form part of the subsequent polymerization stage i (a corresponding procedure can also be followed in the case of two or more polymerization stages i).
The term glass transition temperature as used in this document refers to the glass transition temperature determined by the DSC technique (differential scanning calorimetry, 20xc2x0 C./min, midpoint; cf. ASTM D 3418-82).
It is advantageous in accordance with the invention for Tg1 frequently to be from xe2x88x9260xc2x0 C. to +110xc2x0 C. In other words, exemplary embodiments of the novel process are those where Tg1 is from xe2x88x9250xc2x0 C. to +50xc2x0 C., or where Tg1 is from xe2x88x9250xc2x0 C. to +20xc2x0 C., or where Tg1 is from xe2x88x9250xc2x0 C. to 0xc2x0 C. or where Tg1 is from xe2x88x9240xc2x0 C. to xe2x88x9220xc2x0 C.
The Tg1 of polymers which can be used as pressure-sensitive adhesives is in general in the range from xe2x88x9270xc2x0 C. to 20xc2x0 C., preferably from xe2x88x9260xc2x0 C. to 0xc2x0 C., in particular from xe2x88x9255xc2x0 C. to xe2x88x9220xc2x0 C. and, with particular preference, from xe2x88x9250xc2x0 C. to xe2x88x9220xc2x0 C.
With a given Tgi for the i-th polymerization stage, a monomer composition to be polymerized in the i-th polymerization stage can be assembled in a simple manner using the Fox equation. According to Fox (T. G. Fox, Bull. Am. Phys. Soc. (Ser. II) 1, 123 (1956) and Ullmanns Encyklopxc3xa4die der technischen Chemie, Verlag Chemie, Weinheim, 1980, Vol. 19, 4th edition, p. 18), it holds in good approximation for the high-molecular limit value of the glass transition temperature of random copolymers that             1      Tg        =                            X          1                          Tg          1                    +                        X          2                          Tg          2                    +              …        ⁢                  xe2x80x83                ⁢                              X            n                                Tg            n                                ,
where X1, X2, . . . , Xn are the mass fractions of the monomers 1, 2, . . . , n and Tg1, Tg2, . . . , Tgn are the high-molecular limit values of the glass transition temperatures of homopolymers composed in each case of only one of the monomers 1, 2, . . . or n, in kelvins.
The high-molecular limit values of the glass transition temperatures for the homopolymers are known for the majority of monomers and are listed, for example, in Ullmann""s Encyclopedia of Industrial Chemistry, VCH Weinheim, 1992, fifth edition, Vol. A21, p. 169. Other sources of homopolymer glass transition temperatures are J. Brandrup, E. H. Immergut, Polymer Handbook 1st Ed. J. Wiley, New York 1966, 2nd Ed. J. Wiley, New York 1975, and 3rd Ed. J. Wiley, New York 1989, for example.
Experimentally, random copolymerization of a monomer composition i in a polymerization stage i can essentially be carried out by polymerizing a corresponding monomer mixture by the method of free-radical aqueous emulsion polymerization in the feed technique. In other words, the monomer mixture is pre-emulsified in the aqueous phase and, with the addition of initiators, the mixture is run into the polymerization vessel at the rate at which it is consumed in such a way that the degree of polymerization of the monomers already present in the polymerization vessel is always xe2x89xa795 mol-%. Suitable initiators are preferably sodium peroxodisulfate, and the polymerization is normally carried out at from 60 to 90xc2x0 C. Dispersants which can be employed are the substances recommended in this document for preparing the novel aqueous polymer dispersions. The molecular weight can be regulated in a familiar manner by using substances which have this effect (for example mercaptans) and/or by the amount of initiator used. In the absence of molecular weight regulators and with the use of from 0.1 to 2% by weight of polymerization initiator, based on the amount of monomers, an aqueous polymer dispersion can be obtained whose glass transition temperature corresponds to the high-molecular limit Tgi.
In accordance with the invention, for each Tgi the difference Tgixe2x88x92Tg1 must be at least 10xc2x0 C. In other words, exemplary embodiments of the novel process are those wherein, for each polymerization stage i, the difference Tgixe2x88x92Tg1 is from 10xc2x0 C. to 220xc2x0 C., or from 30xc2x0 C. to 200xc2x0 C., or from 50xc2x0 C. to 180xc2x0 C., or from 80xc2x0 C. to 150xc2x0 C., or from 100xc2x0 C. to 130xc2x0 C.
Where the difference Tgixe2x88x92Tg1 is comparatively small for all polymerization stages i, the novel process brings about, in particular, a pronounced increase in the elongation at break. Where the difference Tgixe2x88x92Tg1 is comparatively high for all polymerization stages i, the novel process brings about, in particular, a pronounced increase in the tear strength. It is advantageous, according to the invention, if the differences Tgixe2x88x92Tg1 are from 60 to 190xc2x0 C. For the use as contact adhesives, Tgixe2x88x92Tg1 of from 60 to 160xc2x0 C. are especially advantageous.
What has been said above is particularly appropriate when, in the novel process, the preparation of the polymer 1 is followed by only one polymerization stage i.
The weight-average diameter {overscore (d)}w, of the polymer 1 present in disperse distribution in the aqueous medium can in accordance with the invention extend over a wide range. For example, {overscore (d)}w can be from 50 nm to 2000 nm. In many cases, {overscore (d)}w will be from 100 nm to 1000 nm or from 200 nm to 500 nm.
Similarly, the number-average molecular weight {overscore (M)}n of the polymer 1 can cover a wide range of values. For example, {overscore (M)}n can be from 5000 or 50,000 to 5xc2x7106 or from 100,000 to 3xc2x7106 or from 250,000 to 1xc2x7106. To prepare the polymer 1 it is of course possible to use molecular weight regulators such as mercaptans (mercaptoethanol, tert-dodecyl mercaptan etc.) and/or halogenated hydrocarbons (e.g. methylene chloride, carbon tetrachloride, chloroform, bromo form, dichloroethane, trichloroethane) and/or C1 to C4 alcohols such as methanol and allyl alcohol. The same applies to the polymerization stages i. Preferably, both the polymer 1 and the polymerization stages i are realized without the use of molecular weight regulators.
In accordance with the invention, the total amount of the monomers polymerized in all polymerization stages i, based on the amount of all of the monomers polymerized in the course of the novel process, must be from 0.1% by weight to  less than 15% by weight, particularly preferably from 0.1 to xe2x89xa610% by weight. In many cases a correspondingly based total amount of monomers polymerized in all polymerization stages i of from 0.5% by weight to 8% by weight is advantageous, or from 1% by weight to 6% by weight or from 2% by weight to 6% by weight.
Monomers suitable for preparing the polymer 1 and for polymerization in the polymerization stages i are, in accordance with the invention and in principle, all monoethylenically unsaturated monomers which can be polymerized by a free-radical mechanism known per se. These are monomers such as styrene, xcex1-methylstyrene, o-chlorostyrene, xcex1-phenylstyrene or vinyltoluenes, esters of vinyl alcohol and C1-C18 monocarboxylic acids, such as vinyl acetate, propionate, n-butyrate, laurate and stearate, esters of allyl alcohol and C1-C18 monocarboxylic acids, such as allyl acetate, propionate, n-butyrate, laurate and stearate, esters of preferably C3-C6 xcex1,xcex2-monoethylenically unsaturated mono- and dicarboxylic acids, especially acrylic, methacrylic, maleic, fumaric and itaconic acid, with in general C1-C12, preferably C1-C8 and, in particular C1-C4 alkanols, such as methyl acrylate and methacrylate, ethyl acrylate and methacrylate, n-butyl acrylate and methacrylate, isobutyl acrylate and methacrylate, tert-butyl acrylate and methacrylate, hexyl acrylate and methacrylate, n-octyl acrylate and methacrylate, 2-ethylhexyl acrylate and methacrylate, decyl acrylate, lauryl acrylate, stearyl acrylate, isobornyl acrylate and methacrylate, norbornyl acrylate, 4-tert-butylcyclohexyl acrylate and methacrylate, 3,3,5-trimethylcyclohexyl acrylate and methacrylate, dimethyl maleate or n-butyl maleate, and also nitrites of xcex1,xcex2-monoethylenically unsaturated carboxylic acids, such as acrylonitrile and methacrylonitrile. Examples of suitable C4-8 conjugated dienes are 1,3-butadiene and isoprene. Also of importance are commercially available monomers VEOVA(copyright) 9-11 (VEOVA X is a tradename of Shell and stands for vinyl esters [of carboxylic acids, which are also known as Versatic(copyright) X acids] of the formula 
in which R1, R2 and R3 are alkyls whose total number of carbons (R1+R2+R3) is equal to X minus 2).
The abovementioned monomers generally account for the majority of the polymer 1 and of the polymerization stages i. In general, the majority accounts for more than 75% by weight of the polymer 1 or of the respective polymerization stage i. Monomers which when polymerized alone usually give homopolymers of increased solubility in water are normally present, both in the polymer 1 and in the polymerization stages i, only in modifying amounts, normally less than 25% by weight, generally from 0.1 to 10% by weight, or from 0.1 to 5% by weight.
Examples of such monomers are C3-C6 xcex1,xcex2-monoethylenically unsaturated mono- and dicarboxylic acids and their amides, for example acrylic, methacrylic, maleic, fumaric and itaconic acid, acrylamide and methacrylamide, and also vinylsulfonic acid, acrylamidopropanesulfonic acid and water-soluble salts of the abovementioned acids. Other examples of such monomers are monoesters of dihydric alcohols with C3-C6 xcex1,xcex2-monoethylenically unsaturated carboxylic acids, such as hydroxyethyl acrylate, n-hydroxypropyl acrylate, n-hydroxybutyl acrylate and the corresponding methacrylates. Normally, copolymerization of the abovementioned monomers brings about increased stability of the disperse distribution of the resulting polymer in the aqueous medium.
The polymerization stages i preferably embrace none of the abovementioned monomers having increased solubility in water. Rather, the monomers to be polymerized in the polymerization stages i are advantageously those having particularly low solubility in water. In general, the only monomers polymerized in the polymerization stages i are those whose molal solubility in water (at 1 atm and 25xc2x0 C.) is less than or equal to the corresponding solubility of methyl methacrylate in water.
In accordance with the invention the polymer 1 can also contain in copolymerized form monomers containing 2 or more nonconjugated ethylenically unsaturated groups, for example the diesters of dihydric alcohols with xcex1,xcex2-monoethylenically unsaturated monocarboxylic acids. Preferred diunsaturated diesters are those of acrylic and of methacrylic acids. Examples which may be given are alkylene glycol diacrylates and dimethacrylates, such as ethylene glycol diacrylate, 1,3-butylene glycol diacrylate, 1,4-butylene glycol diacrylate, propylene glycol diacrylate, 1,6-hexanediol diacrylate, dipropylene glycol diacrylate, tripropylene glycol diacrylate and trimethylolpropane triacrylate. As crosslinking constituents of the polymer 1, monomers such as divinylbenzene, vinyl methacrylate, vinyl acrylate, allyl methacrylate, allyl acrylate, diallyl maleate, diallyl fumarate, methylenebisacrylamide, cyclopentadienyl acrylate and triallyl cyanurate are also suitable.
In many cases it is advantageous if the polymer 1 and/or one or more of the polymerization stages i possess in minor amounts, generally in amounts of from 0.5 to 5% by weight, those monomers which bring about crosslinking only in the course of film formation. Examples that may be mentioned are carbonyl-containing monomers such as acrolein, methacrolein, diacetoneacrylamide and -methacrylamide, and also vinyl acetoacetate. The abovementioned monomers bring about post-crosslinking, for example, when the aqueous polymer dispersion contains at the same time an appropriate amount of an added polyamine compound. Particularly suitable such compounds are the dihydrazides of C2-C10 aliphatic dicarboxylic acids. Examples are the dihydrazides of oxalic, malonic, succinic, glutaric, adipic and sebacic acids.
An example of another monomer which brings about post-crosslinking is 2-acetoacetoxyethyl methacrylate (alone or in combination with polyamines or polyaldehydes such as glyoxal). Also suitable for post-crosslinking are those polymer units which have hydrolyzable Si-organic bonds. Examples which may be mentioned are the copolymerizable monomers methacryloyloxypropyltrimethoxysilane and vinyltrimethoxysilane. Other suitable polymer units of corresponding type are given in DE-A 43 41 260. If the disperse polymer particles have carboxyl groups, then post-crosslinking can also be brought about by adding metal salts containing polyvalent cations (e.g. Mg, Ca, Zn or Zr salts). Also suitable for the purpose of post-crosslinking are monomers containing epoxy and/or N-alkylol groups, for example glycidyl acrylate, N-methylolacrylamide and N-methylolmethacrylamide. Post-crosslinking can also be achieved by copolymerization of small amounts of unsaturated benzophenone derivatives or acetophenone derivatives, and subsequent photoinitiation. As an alternative, appropriate saturated benzophenone derivatives or acetophenone derivatives can also be stirred into the aqueous polymer dispersions obtainable in accordance with the invention.
It should be stated, however, at this point that the invention does not require the use of post-crosslinking constituents in order to obtain the desired mechanical property profile of the dispersion polymer films. Where the novel process comprises systems which bring about post-crosslinking, then the glass transition temperatures Tg1 and Tgi refer to the glass transition temperatures that are found when these crosslinking constituents, present only in minor quantities, are excluded.
In addition, the polymer 1 and/or the polymerization stages i may include minor amounts, generally from 0.1 to 5% by weight, of adhesion monomers (e.g. nitrogen-containing monomers), in order to increase the adhesion of the film of the resulting aqueous polymer dispersion to numerous materials such as wood, metal, minerals, paper, textiles and plastic, but in particular to old coatings based on drying oils and/or alkyd resins, and to reduce the sensitivity of this adhesion to the effects of moisture and wetness (increased wet adhesion).
Particularly suitable nitrogen-containing adhesion monomers are free-radically polymerizable monomers having at least one amino, ureido or N-heterocyclic group. A large number of such suitable adhesion monomers is given in EP-B 421 185, in EP-B 379 892 on page 3, in EP-A 609 756 on page 2, in DE-A 43 34 178 and in DE-A 39 02 067 on pages 3/4.
Examples which may be mentioned are aminoethyl acrylate and methacrylate, dimethylaminoethyl acrylate and methacrylate, diethylaminoethyl acrylate and methacrylate, dimethylaminopropyl acrylate and methacrylate, 3-dimethylamino-2,2-dimethylpropyl acrylate and methacrylate, 2-N-morpholinoethyl acrylate and methacrylate, 2-N-piperidinoethyl acrylate and methacrylate, N-(3-dimethyl-aminopropyl)acrylamide and -methacrylamide, N-dimethylaminoethylacrylamide and -methacrylamide, N-diethylaminoethylacrylamide and -methacrylamide, N-(4-morpholinomethyl)acrylamide and -methacrylamide, vinylimidazole and also monoethylenically unsaturated derivatives of ethyleneurea, such as N-(2-acryloyloxyethyl)ethyleneurea, N-(xcex2-acrylamidoethyl)ethyleneurea, N-2-(allylcarbamato)aminoethylimidazolidinone, N-(3-allyloxy-2-hydroxypropyl)aminoethylethyleneurea, N-vinylethyleneurea, N-vinyloxyethyleneurea, N-methacryloyloxyacetoxyethylethyleneurea, N-(acrylamidoethylene)ethyleneurea, N-(methacrylamidoethylene)ethyleneurea, 1-(2-methacryloyloxyethyl)imidazolin-2-one and N-(methacrylamidoethyl)ethyleneurea. Further suitable ureido monomers are given in a review article by R. W. Kreis, A. M. Sherman, xe2x80x9cDevelopments in Ureido Functional Monomer for Promoting Wet Adhesion in Latex Paintsxe2x80x9d, Water-Borne and Higher Solids Coating Symposium, from Feb. 3 to 5, 1988, New Orleans, La.
The novel process is enhanced if the monomers used to construct the polymer 1 and the monomers that are polymerized in the polymerization stages i are chemically similar. In other words, if the polymer 1 is composed predominantly of alkyl esters of acrylic and methacrylic acid or of a mixture thereof with styrene and/or acrlyonitrile, it is advantageous if in the polymerization stages i as well, predominantly, alkyl esters of acrylic and methacrylic acid are copolymerized, either alone or in a mixture with styrene and/or acrylonitrile. If, on the other hand, the polymer 1 is polymerized predominantly from vinyl esters or a mixture thereof with ethylene, it is advantageous if predominantly vinyl esters or a mixture thereof with ethylene are polymerized in the polymerization stages i as well. Where the polymer 1 consists predominantly of butadiene or of a mixture thereof with acrylonitrile and/or styrene, it is advantageous if acrylonitrile and/or styrene also form the principal constituents of the polymerization stages i. In other words, in the context of the novel procedure, at least 75% by weight of the composition of the polymer 1 can be accounted for only by esters of acrylic and/or methacrylic acid with C1- to C8-alkanols or by a mixture thereof with styrene and/or acrylonitrile. Also conceivable, however, is a composition of which at least 75% is accounted for by vinyl esters and/or ethylene.
Accordingly, the novel process can be employed in cases where from 90 (preferably from 95) to 100% by weight of the composition of polymers 1, in free-radically polymerized form, is accounted for by esters of acrylic and/or methacrylic acid with C1- to C8-alkanols and from 0 to 10 (preferably to 5) % by weight by C3-C6 xcex1,xcex2-monoethylenically unsaturated carboxylic acids, their amides and/or alkali metal salts.
However, it can also be employed in the case where from 95 to 100% by weight of the composition of polymers 1, in free-radically polymerized form, is accounted for by esters of acrylic and/or methacrylic acid with C1-C8-alkanols and styrene, and from 0 to 5% by weight by C3-C6 xcex1,xcex2-monoethylenically unsaturated carboxylic acids, their amides and/or alkali metal salts.
The monomers for the construction of the polymer 1 are preferably from the group consisting of ethyl acrylate, methyl methacrylate, n-butyl acrylate, 2-ethylhexyl acrylate, n-butyl methacrylate, acrylic acid, methacrylic acid, acrylamide, methacrylamide, itaconic acid, styrene, acrylonitrile, acrylamidopropanesulfonic acid and vinylsulfonic acid, and, particularly preferably, from the group consisting of n-butyl acrylate, methyl methacrylate, 2-ethylhexyl acrylate, n-butyl methacrylate, acrylic acid, methacrylic acid, acrylamide, methacrylamide and itaconic acid.
In all of the abovementioned cases, isobutyl methacrylate, n-butyl methacrylat, tert-butyl methacrylate, isobornyl acrylate, isobornyl methacrylate, 4-tert-butylcyclohexyl acrylate, 4-tert-butylcyclohexyl methacrylate, 3,3,5-trimethylcyclohexyl acrylate, 3,3,5-trimethylcyclohexyl methacrylate, acrylonitrile, styrene, xcex1-phenylstyrene, xcex1-methylstyrene, methyl methacrylate and mixtures of the abovementioned monomers constitute the monomers which are preferably polymerized in the polymerization stages i. This statement also applies in the case of other polymers 1.
To prepare polymer dispersions which can be used as pressure-sensitive adhesives use is made, for the polymer 1, preferably of at least one monomer whose homopolymer has a Tg less than 0xc2x0 C. and is selected from esters of acrylic acid and methacrylic acid with an alkanol having at least 4 carbons, especially n-butyl acrylate, isobutyl acrylate, hexyl acrylate, n-octyl acrylate, 2-ethylhexyl acrylate, 2-ethylhexyl methacrylate, decyl acrylate, lauryl acrylate, stearyl acrylate, isobornyl acrylate and other monomers having homopolymer Tgs less than 0xc2x0 C. Particular preference is given to n-butyl acrylate and/or 2-ethylhexyl acrylate. These monomers form the main fraction, that is they are employed in general in amounts of from 70 to 100% by weight, in particular from 80 to 100% by weight, based on the overall weight of the monomers used to prepare the polymer 1. The polymer 1 can, consequently, be composed of up to 30% by weight, in particular up to 20% by weight, of comonomers having a Tg greater than 0xc2x0 C. These are preferably selected from styrene, xcex1-methylstyrene, o-chlorostyrene, xcex1-phenylstyrene, vinylsulfonic acid, acrylamidopropanesulfonic acid, acrylonitrile, methacrylonitrile, acrylic acid, methacrylic acid, acrylamide, methyl acrylate, methyl methacrylate, ethyl acrylate, tert-butyl acrylate, tert-butyl methacrylate, vinyl acetate, vinyl propionate, hydroxyethyl acrylate, hydroxypropyl acrylate, n-butyl methacrylate and methacrylamide, particular preference being given to (meth)acrylic acid and the amides thereof. What has been said above applies to the monomers polymerized in the stages i, in other words they are subject to no specific restriction.
In accordance with the invention, the polymer 1 is prepared by the method of free-radical aqueous emulsion polymerization, ie. in the presence of dispersant and free-radical polymerization initiator, directly in the aqueous medium, in disperse distribution.
Suitable dispersants include both the protective colloids which are customarily employed for conducting free-radical aqueous emulsion polymerizations, and emulsifiers.
Examples of suitable protective colloids are polyvinyl alcohols, cellulose derivatives or copolymers containing vinylpyrrolidone.
A detailed description of further suitable protective colloids is given in Houben-Weyl, Methoden der organischen Chemie, Band XIV/1, Makromolekulare Stoffe, Georg-Thieme-Verlag, Stuttgart, 1961, pp. 411-420. It is of course also possible to use mixtures of emulsifiers and/or protective colloids. As dispersants it is preferred to employ exclusively emulsifiers, whose relative molecular weights, unlike those of the protective colloids, are usually below 2000, preferably below 1000. They can be anionic, cationic or nonionic in nature. In the case where mixtures of surfactants are used, the individual components must of course be compatible with one another; in case of doubt, this can be checked by means of a few experiments beforehand. In general, anionic emulsifiers are compatible with one another and with nonionic emulsifiers. The same applies to cationic emulsifiers, whereas cationic and anionic emulsifiers are usually incompatible with one another. Examples of common emulsifiers are ethoxylated mono-, di and trialkylphenols (EO units: 3 to 100, alkyl: C4 to C12), ethoxylated fatty alcohols (EO units: 3 to 100, preferably 6 to 50, alkyl: C6 to C20), and also alkali metal and ammonium salts of alkyl sulfates (alkyl: C8 to C18), of sulfuric acid monoesters of ethoxylated alkanols (EO unit: 1 to 70, especially 2 to 10, alkyl: C10 to C18) and ethoxylated alkylphenols (EO units: 3 to 100, preferably 6 to 50, alkyl: C4 to C18), of alkylsulfonic acids (alkyl: C10 to C18) and of alkylarylsulfonic acids (alkyl: C9 to C18). Other suitable emulsifiers, such as sulfosuccinic esters, are given in Houben-Weyl [loc.cit.], Volume XIV/1, pp. 192-208.
Other compounds which have been found to be suitable surfactants are those of the formula I 
in which A1 and A2 are hydrogen or C4- to C24-alkyl but are not both simultaneously hydrogen, and X and Y can be alkali metal ions and/or ammonium ions. In the formula I, A1 and A2 are preferably linear or branched alkyls of 6 to 18 carbons or hydrogen, and in particular have 6, 12 or 16 carbons, and are not both simultaneously hydrogen. X and Y are preferably sodium, potassium or ammonium ions, sodium being particularly preferred. Particularly advantageous compounds I are those in which X and Y are sodium, A1 is a branched alkyl of 12 carbons and A2 is hydrogen or the same as A1. Use is frequently made of technical-grade mixtures containing from 50 to 20% by weight of the monoalkylated product, an example being Dowfax(copyright) 2A1 (trademark of DOW Chemical Company). The compounds I are frequently employed on their own or in a mixture with ethoxylated fatty alcohols (EO units: 3 to 50, alkyl: C8 to C36) as dispersants. The compounds I are generally known, for example from U.S. Pat. No. 4,269,749, and are obtainable commercially. In general the amount of dispersant employed is from 0.5 to 6% by weight, preferably from 1 to 5% by weight and, with particular preference, from 2 to 4% by weight, based on the monomers that are to be subjected to free-radical polymerization in order to produce the polymer 1.
The initiation of the free-radical polymerization to prepare the polymer 1 can in principle be accomplished by all free-radical initiators which are capable of triggering a free-radical aqueous emulsion polymerization. They can be peroxides, for example H2O2, peroxodisulfuric acid and/or salts thereof, for example alkali metal or ammonium peroxodisulfate, and also azo compounds, such as azobisisobutyronitrile or 4,4xe2x80x2-azobiscyanovaleric acid.
Initiation of the free-radical aqueous emulsion polymerization to prepare the polymer 1 at comparatively low temperatures is made possible by combined initiator systems, which are composed of at least one organic and/or inorganic reducing agent and of at least one peroxide and/or hydroperoxide, examples being tert-butyl hydroperoxide and the sodium metal salt of hydroxymethanesulfinic acid, or hydrogen peroxide and ascorbic acid. What has been said above applies in particular to combined systems which additionally comprise a small amount of a metal compound which is soluble in the polymerization medium and whose metal component can exist in two or more valency states (in many cases in complexed form, in order to prevent precipitation), an example being ascorbic acid/iron(II) sulfate/hydrogen peroxide, it being possible for the ascorbic acid to be replaced, for example, by the sodium metal salt of hydroxymethanesulfinic acid, sodium sulfite or sodium hydrogen sulfite, and the hydrogen peroxide by, for example, alkali metal peroxodisulfate and/or ammonium peroxodisulfate. Instead of a water-soluble iron(II) salt it is also possible to use a V salt or a combination of water-soluble Fe/V salts. In general, the amount of free-radical initiator system used to prepare the polymer 1, based on the amount of the monomers to be polymerized, is from 0.1 to 2% by weight.
In the context of the preparation of the polymer 1, the temperature and pressure of polymerization are of relatively minor importance. Polymerization is generally carried out at from 0 to 100xc2x0 C., frequently from 20xc2x0 C. to 100xc2x0 C., or at from 50 to 95xc2x0 C., usually from 60 to 90xc2x0 C. In order to obtain particularly long-chain polymers 1 polymerization will expediently be carried out at from 40 to 70xc2x0 C. The use of sub- or superatmospheric pressure is possible, and so the polymerization temperature may exceed 100xc2x0 C. and can be up to 130xc2x0 C. or more. Highly volatile monomers such as ethylene or butadiene are preferably polymerized under superatmospheric pressure. To regulate the pH of the polymerization medium, it is possible to add buffers, such as NaHCO3, Na2CO3, Na-acetate or Na2P2O5, during the free-radical aqueous emulsion polymerization to prepare the polymer 1.
To improve the reproducibility and establishment of defined particle diameters it is possible, in the context of the preparation of the polymer 1, to decouple the polymer particle formation phase and the polymer particle growth phase from one another in a manner known per se to the skilled worker by charging to the polymerization vessel a defined amount of a preformed aqueous polymer dispersion (a seed latex) or preforming such a dispersion in situ in said vessel. The amount of dispersant added in the subsequent course of the free-radical aqueous emulsion polymerization is then in general calculated so as not to exceed the critical micelle concentration and so as largely to avoid the formation of new polymer particles. If a broad distribution of the diameters of the particles of the polymer 1 is desired, the seed latex will generally be added to the polymerization vessel, in a manner known per se, during the free-radical aqueous emulsion polymerization. Like free-radical polymerization processes in general, both the free-radical aqueous emulsion polymerization to prepare the polymer 1 and each polymerization stage i can be carried out under an inert gas atmosphere (eg. N2, Ar) or under an oxygen-containing atmosphere (eg. air)
The free-radical aqueous emulsion polymerization to prepare an aqueous dispersion of a polymer 1 can with particular simplicity be carried out as follows.
The monomers to be polymerized are emulsified in the aqueous medium, and the free-radical polymerization initiator is dissolved in water. Water and, if desired, a small amount of xcex1,xcex2-monoethylenically unsaturated carboxylic acid are charged to the polymerization vessel, and this initial charge is heated to the polymerization temperature. Then part of the aqueous monomer emulsion and part of the aqueous initiator solution are added in one go to the initial charge, which is then polymerized. Subsequently, while maintaining the polymerization, the remainder of the aqueous monomer emulsion and the remainder of the aqueous initiator solution are supplied continuously to the polymerization vessel, preferably with substantial synchronicity. After the end of the supply of initiator and monomer, the polymerization mixture will expediently be left alone for some time, with stirring and while maintaining the polymerization temperature. To conduct the subsequent polymerization stages i, the reaction mixture will then expediently first of all be cooled. The monomers to be polymerized in the stage i will then be added to the polymerization vessel, preferably on their own and all at once. The emulsifier contents of the aqueous dispersion of the polymer 1 here will advantageously be such that the aqueous dispersion medium contains essentially no emulsifier micelles. The reactor contents, which advantageously would have been cooled, will generally then expediently be left alone for a certain period, with stirring, in order to promote swelling. Subsequently, fresh free-radical polymerization initiator will normally be added, and the batch will then be heated to the polymerization temperature and polymerized as desired. If required, further polymerization stages i can be appended correspondingly.
Free-radical polymerization initiators which can be used to initiate the polymerization stages i are in principle all those which have already been mentioned in connection with the preparation of the dispersion of the polymer 1. Preference, however, is given to employing those free-radical initiator systems that comprise organic peroxides or organic hydroperoxides, especially redox initiator systems comprising such organic peroxides and/or hydroperoxides. Free-radical initiator systems comprising tertbutyl hydroperoxide and cumene hydroperoxide are particularly suitable. However, free-radical initiator systems comprising pinane hydroperoxide, p-menthane hydroperoxide, diisopropylphenyl hydroperoxide, dibenzoyl peroxide, dilauryl peroxide and diacetyl peroxide are also suitable. Examples of suitable reductive partners in such systems are alkali metal sulfites, ascorbic acid, acetone bisulfite, and the alkali metal salts of hydroxymethanesulfinic acid. Based on the monomers to be polymerized in a polymerization stage i, from 0.1 to 2% by weight of free-radical polymerization initiator will generally be added. It is of course also possible, in a less preferred embodiment of the present invention, to add to the polymerization vessel the monomers that are to be polymerized in a polymerization stage i in a form in which they are pre-emulsified in an aqueous medium. Dispersants which can be used in this context are all those already recommended for preparing the aqueous dispersion of the polymer 1. In view of the preferred use of organic redox initiator systems in the polymerization stages i, the associated polymerization temperature will normally be below 80xc2x0 C. (in most cases xe2x89xa740xc2x0 C. to xe2x89xa680xc2x0 C.). As far as the polymerization pressure is concerned, what was said in relation to the preparation of the polymer 1 applies here too.
The films of the aqueous polymer dispersions (pigmented and/or filled, or not pigmented and not filled) that result in accordance with the invention have not only the advantages already described but also good flexibility, reduced surface tack and satisfactory hardness. They are suitable, inter alia, as binders for coatings on leather and on hydrophobic substrates, for example on flat roofs which for reasons, for instance, of heat insulation are coated, for example, with polyurethane-based compositions. By employing one of the known spray-drying techniques it is possible to convert the aqueous polymer dispersions that are obtainable in accordance with the invention to polymer powders that can be redispersed in an aqueous medium. Converted to this dry form, the aqueous polymer dispersions obtainable in accordance with the invention can be transported at favorable cost before being converted back to the aqueous dispersion form at the site of use. In addition, the polymer powders thus obtainable can be used to produce market-oriented dry plasters and to produce polymer-modified binding mineral building materials, such as dry mortars, especially these based on cement. The polymer systems obtainable it accordance with the invention are also suitable as adhesives, as binders for paper coating slips and as binders for nonwovens. In particular, they are suitable as already mentioned for crackbridging coating compositions (for example paints).
Coating compositions of this kind generally contain from 40 to 95% by weight, preferably from 60 to 90% by weight and, in particular, from 65 to 85% by weight of nonvolatile constituents. Based on their total amount, from about 10 to 99.5% by weight, preferably from 10 to 50% by weight and, in particular, from 20 to 40% by weight of these constituents are accounted for by the solids present in the polymeric binder, from 0 to 60% by weight, preferably from 20 to 60% by weight, in particular from 30 to 55% by weight by fillers, from 0 to 60% by weight, preferably from 0 to 20% by weight by pigments, and from 0.5 to 25% by weight, preferably from 1 to 10% by weight, by auxiliaries.
Examples of suitable fillers are alumosilicates, silicates, alkaline earth metal carbonates, preferably calcium carbonate in the form of calcite or chalk, dolomite, and also aluminum silicates or magnesium silicates such as talc.
An example of a typical pigment is titanium dioxide, preferably in the rutile form. However, the coating compositionsxe2x80x94especially when used for decorative purposesxe2x80x94may also include colored pigments, examples being iron oxides.
Customary auxiliaries include wetting agents, such as sodium or potassium polyphosphates, polyacrylic acids, their alkali metal salts, polyvinyl alcohols, etc. In addition these coating compositions generally contain viscosity modifiers such as cellulose ethers, an example being hydroxyethylcellulose. In addition, the coating compositions can have added to them dispersants, antifoams, preservatives, hydrophobicizing agents, biocides, dyes, fibers or other constituents. In order to establish the film-forming properties of the binder polymers, the coating compositions may also include plasticizers.
The polymer present in the aqueous polymer dispersions that are obtainable in accordance with the invention can of course also be isolated by coagulation, which can be carried out in a manner known per se, and this polymer can be used, for example, to modify engineering plastics.
The aqueous polymer dispersions obtainable in accordance with the invention can also be used to modify other aqueous polymer dispersions. This can be done, simply, by adding them to the other aqueous polymer dispersions. An alternative, however, is to use them as a seed dispersion for preparing a different aqueous polymer dispersion.
In addition, the novel polymers can be used as adhesives, especially pressure-sensitive adhesives. They are particularly suitable as pressure-sensitive adhesives.
For this purpose the polymers are preferably used in the form of their aqueous dispersion.
In the case of utility as pressure-sensitive adhesive, a tackifier, in other words a resin which provides tack, can be added to the polymers or to the aqueous dispersions of the polymers. Tackifiers are known, for example, from Adhesive Age, July 1987, page 19-23 or Polym. Mater. Sci. Eng. 61 (1989), page 588-592.
Examples of tackifiers are natural resins, such as rosins and their derivatives produced by disproportionation or isomerization, polymerization, dimerization and/or hydrogenation. They can be present in their salt form (for example with mono- or polyvalent counterions (cations)) or, preferably, in their esterified form. Alcohols used for the esterification-can be monohydric or polyhydric. Examples are methanol, ethanediol, diethylene glycol, triethylene glycol, 1,2,3-propanetriol or pentaerythritol.
Also used, furthermore, as tackifiers are hydrocarbon resins, for example indene-coumarone resins, polyterpene resins, hydrocarbon resins based on unsaturated CH compounds, such as butadiene, pentene, methylbutene, isoprene, piperylene, divinylmethane, pentadiene, cyclopentene, cyclopentadiene, cyclohexadiene, styrene, xcex1-methylstyrene or vinyltoluene.
Other compounds increasingly being used as tackifiers are polyacrylates of low molar weight. These polyacrylates preferably have a weight-average molecular weight Mw of below 30,000. At least 60, in particular at least 80% by weight of the polyacrylates preferably consists of C1-C8 alkyl (meth)acrylates.
Preferred tackifiers are natural or chemically modified rosins. Rosins consist primarily of abietic acid or derivatives thereof.
The tackifiers can be added in a simple manner to the novel polymers, preferably to the aqueous dispersions of the polymers. In this case the tackifiers are preferably themselves in the form of an aqueous dispersion.
The amount by weight of the tackifiers is preferably from 5 to 100 parts by weight, particularly preferably from 10 to 50 parts by weight, based on 100 parts by weight of polymer (solids/solids).
In addition to tackifiers it is also possible to use additives, for example thickeners, antifoams, plasticizers, pigments, wetting agents or fillers, in the case of utility of the polymers as (pressure-sensitive) adhesives.
For use as a (pressure-sensitive) adhesive the novel polymers and aqueous dispersions or aqueous formulations can be applied by customary methods, for example by rolling, knife coating, spreading, etc., to substrates such as paper or polymer films, preferably consisting of polyethylene, polypropylene, which can be coaxially or monoaxially oriented, polyethylene terephthalate, polyvinyl chloride, polystyrene, polyamide or metal. The water can be removed preferably by drying at from 50 to 150xc2x0 C. For subsequent use, the side of the substrates, for example the labels, that is coated with the (pressure-sensitive) adhesive can be covered with a release paper, for example with a siliconized paper.
When used as adhesives the novel polymers exhibit improved cohesion coupled with essentially unchanged adhesion and tackiness.